Bump mapserver_mod from `b4ffb0a` to `594e5da`

Bumps [mapserver_mod](https://github.com/minetest-mapserver/mapserver_mod) from `b4ffb0a` to `594e5da`.
- [Release notes](https://github.com/minetest-mapserver/mapserver_mod/releases)
- [Commits](https://github.com/minetest-mapserver/mapserver_mod/compare/b4ffb0a0587bae989adf476ac8d44091659cfd55...594e5dad565c75c5c46292378ce17fb5d473d2c1)

mapserver_mod @ 594e5dad

-Subproject commit b4ffb0a0587bae989adf476ac8d44091659cfd55
+Subproject commit 594e5dad565c75c5c46292378ce17fb5d473d2c1